In the course of the normal service of same, metallic parts such as brake drums are subjected to dry, unlubricated sliding wear and high frictional loads. These components must absorb energy between the sliding surfaces while maintaining their structural integrity (resistance to cracking) and while exhibiting reasonably predictable wear rates (resistance to scuffing and galling). Because of their physical and mechanical properties, as well as their unique microstructures, graphitic cast irons, particularly so-called gray irons, are commonly used in these applications. For purposes of this specification, we intend by use of the term "graphitic cast iron" to mean a family of cast ferrous alloys having a carbon content of at least 2%, most of which is uncombined and present in the form of free graphite. Further, as used herein, the term "gray irons" refers to specific types of graphitic cast irons in which the graphite is present in the form of flakes and which derive their name from the gray appearance of their fracture surfaces.
Under moderate operating conditions, acceptable wear rates are experienced due to adhesion and microscopic metal transfer between the contacting sliding surfaces.
However, in heavy duty service, significant amounts of metal transfer, smearing and severe surface damage, described as "scuffing and galling" may occur. The cyclic frictional heating and cooling of the sliding surfaces which accompanies such wear can also cause a network of fine, thermal fatigue cracks, termed "heat checks", to develop in the surfaces. Propagation of these cracks through the cross-section can cause the catastrophic fracture of a brake drum or similar structure. Studies have shown that scuffing and heat checking in graphitic cast irons are intimately linked to the surface properties of the components.
In practice, the wear properties, performance and life of graphitic cast iron brake drums, particularly where such drums are subjected to the heavy duty use that is incident, for example, to operation of large trucks, has been largely limited by metal impairing phenomena of the type which are above discussed. While various methodology has been considered in an effort to limit the effects of such phenomena, by and large it has been considered up to the present that the practical limit of improvement for such cast iron drums has been reached, and that the wear characteristics could not be practically extended.
In accordance with the foregoing, it may be considered an object of the present invention, to provide a method for surface treating a graphitic cast iron brake drum, which is effective in increasing the resistance of the cast iron surface to scuffing and heat checking under conditions of dry sliding contact and frictional loading.
It is a further object of the present invention, to provide a method of the foregoing character which can be carried out with existing technology and apparatus, and at fully acceptable costs.
A yet further object of the present invention is to provide a graphitic cast iron brake drum, the brake shoe engaging surfaces of which have been cold worked as to yield a product having increased resistance to scuffing and heat checking during the subsequent veh2icular use of the treated brake drum.